1. Field of the Invention
This invention pertains to pipe laying vessels and, more particularly, to constant tension winch/pipe handling systems on board such vessels.
2. Description of the Prior Art
Pipe laying vessels, usually in the form of barges, are maintained in accurate navigational positions generally by a plurality of anchors on the ocean floor or, in some cases, by computerized propulsion systems aboard the vessel itself. These vessels are extremely expensive to manufacture. As a necessary incidence to the laying of pipe from a vessel, one or more pipe tensioners, welders, and other pipe handling and storage equipment must be provided on top of the deck of the vessel. Each additional piece of equipment which must be added above deck has generally required the lengthening of the deck at the high cost.
One accessory piece of equipment necessary on a pipe laying vessel is a constant tension winch for abandonment and recovery of pipe during bad weather conditions. Although the constant tension winch has conventional internal control mechanisms which tend to maintain the tension on the cable and thus on the pipe connected to the cable at a generally constant tension loading, changes in cable loading caused by vessel oscillations from wave and wind actions frequently exceed the stress limit of the cable and the capability of the winch to maintain the constant tension in the pipe.